The California Sea Hare (Aplysia californica) is among the world's largest gastropods and grows to lengths of about 41cm (16in). It has ruffled flaps along its back and two pairs of antennae. The color of an individual Aplysia californica is determined by the color of the algae (or other plant matter) on which it feeds, but in general can be shades of red, brown, or green.

When disturbed, Aplysia californica secretes a purple dye from glands under its mantle (see photo, courtesy Genevieve Anderson). The purpose of the dye secretion is not fully understood. Some researchers have proposed that the dye acts as an 'inky' screen or decoy to deter predators.